The present invention relates to a system for reliably measuring the flow rate of a low temperature vaporization fluid and for regulating fluid flow in response thereto. In a suitable application, the system of the invention may be used by a farmer while fertilizing crops to accurately disperse liquid anhydrous ammonia (NH3) from a nurse tank. An improved and relatively low cost flow meter is provided ideally suited for measuring the flow rate of a low temperature vaporization fluid, such as anhydrous ammonia.
Anhydrous ammonia (NH3), which is 82% nitrogen, is applied to soil by farmers as a fertilizer. Farmers often use a nurse tank containing pressurized liquid NH3 as the source. This tank is transported by the farm vehicle across a field while the NH3 is distributed to the soil. An over application of NH3 costs the farmer money, and an under application affects the crop. Moreover, since groundwater contamination attributable to NH3 has become a more prominent issue (now regulated by some states), it is desirable to accurately control the flow of NH3.
The crudest method of controlling the flow of NH3 to the soil is to partially open a ball valve and roughly calculate the flow rate of NH3 to the soil. This may be done by reading the percentage the tank is full with a meter on the tank. The farmer then makes a test run and, based upon speed, acreage and the amount of NH3 used, he calculates flow. Several test runs followed by valve adjustment may be necessary to achieve the desired flow rate. If the tank pressure gauge indicates a change in pressure during the course of a day as a result of the NH3 warming to the daily outdoor temperature, the flow rate has changed even though the valve position remains fixed. Accordingly, another test run may be needed. Needless to say, this method is crude and burdensome.
More accurate flow measurement for real time flow control of NH3 presents a rather difficult problem. NH3 has a low boiling point (low vaporization temperature). Pressure drops result in flashes (liquid turning to vapor) in the NH3, and the created vapor makes the flow measurement inaccurate. Without an accurate measurement of the flow rate, the farmer cannot properly control the application of NH3. A number of variables can cause the flow rate to change, including the ground speed of the farmer""s vehicle, the temperature within the nurse tank and flow lines, soil density, the desired application, and the flow position of the regulator or valve (ranging from fully closed to fully open). Moreover, the farmer has no control over soil density or the temperature within the nurse tank, which can vary greatly during the course of a day. The prior art teaches that accurate measurement of the NH3 flow rate requires condensation after the NH3 is two phased (liquid/vapor). Heat exchanger and/or NH3 liquifiers for performing this condensation purpose are expensive and require high maintenance.
Although others recognized the problem the farmer experienced in controlling the amount of NH3 to be applied to the soil, the prior art has failed to devise a simple, accurate and inexpensive system for resolving the problem. For over 20 years, prior art systems attempted to obtain a more accurate measurement of the flow rate by taking the two phase NH3, returning it to a single liquid phase, and then measuring the flow rate of this liquid. A continuing problem with such systems are their expense. Moreover, the condenser incorporated into the system never fully converts the two phase NH3 back to liquid, and the condenser inherently uses a restriction in the flow path. The severity of the restriction increases during cold weather or low pressures. The cost of these systems for a typical farmer is commonly prohibitive, or is unjustifiable given the savings to be realized. The condensers are commonly designed for a specified flow rate, and at flow rates exceeding the specified flow rate, the condenser has difficulty converting the vapor to liquid, thereby reducing the accuracy of the flow measurement. In teaching that more accurate flow measurement required the taking of two phase NH3 and returning to a single phase with a heat exchanger, the prior art devices taught away from the present invention.
In most prior art NH3 dispensing systems, the flow meter is the most delicate component. Numerous types of flow meters have been devised, including both variable cross-sectional area flow meters wherein the cross-sectional flow cavity through the flow meter is indicative of flow rate, and turbine flow meters wherein the angular velocity of the turbine is proportional to the flow rate. With regard first to variable area flow meters, it is known that such flow meters may be devised such that the flow rate is related to the position of a member which defines the cross-sectional flow area through the meter at any point in time. Prior art variable area meters have several significant drawbacks, however, which have resulted in these meters not being acceptable for use in measuring the flow rate of anhydrous ammonia. Some of these meters include a sensor mounted on the vane shaft, but a seal is required between the flow chamber and the sensor. One patent disclosing such a meter is U.S. Pat. No. 3,835,373. The seal is subject to a highly hostile environment when the meter is used for fluids such as anhydrous ammonia, and accordingly this type of meter would not generally be considered acceptable for use on an anhydrous ammonia distribution system.
Another type of variable area meter utilizes a magnet mounted on a vertically suspended body and a hall sensor to provide an electronic output of the position of the suspended body and thus the flow rate through the meter. A system of this type is disclosed in U.S. Pat. No. 5,187,988. This meter would typically not be suitable for use in the application discussed above since the meter must be positioned in a true vertical position for proper flow measurement. Many fields commonly have rolling hills, and both the tractor and the equipment pulled by the tractor are thus not always moving truly horizontally. The flow meter discussed in this patent and the vertically suspended body in particular would also be highly susceptible to inaccurate readings and/or damage if subjected to vibration of the type common to farming equipment. This meter is also designed for a very low pressure application, and anhydrous ammonia is typically dispensed at medium or high pressures in excess of 250 psi.
Other variable flow area type devices are disclosed in U.S. Pat. Nos. 5,497,081, 5,444,533 and 5,327,789. Many types of these flow measurement devices are frequently designed to operate in the vertical position. Complicated sensor assemblies are frequently employed to detect the position of the flow area defining member. These complicated detector and sensor assemblies are very costly, and are not highly reliable when used in the rugged environment required for farming equipment. Other variable area meters employ complicated flux concentrators. Mechanical calibration or remote read-out devices which are generally unsuitable for anhydrous ammonia applications are also commonly associated with variable area flow meters, as disclosed in U.S. Pat. No. 4,487,007. Farmers want a meter which has a low cost and which is not complicated or difficult to calibrate. As explained above, many prior art variable area flow meters must be positioned vertically to be accurate, and this restriction is unacceptable to NH3 applications. U.S. Pat. No. 5,444,369 discloses another type of variable area meter. Various pole pieces must be precisely positioned in order to provide a desired linear output between the flow and the electronic output from the hall device. Prior art meters which rely upon a variable area concept for measuring flow have thus long been considered too expensive, too complicated, too delicate, and too limiting for anhydrous ammonia use.
Almost all flow meters currently in commercial use for measuring the flow rate of anhydrous ammonia applied from the nurse tank to the field are of a type which employ a rotating turbine, wherein the rotational output of the shaft is proportional to the flow rate. These turbine-type meters common employ a magnetic pick off on the shaft, so that each rotation of the shaft produces an output signal, the number of pulses or signals generated during any period of time is thus used to determine the flow rate of anhydrous ammonia through the meter. Turbine-type meters are quite expensive, but are generally considered rugged and do not require precise positioning to provide an output. Unfortunately, a significant disadvantage of such meters when used for measuring the flow of fluids which are easily vaporized is that the meters are frequently damaged when the liquid nurse tank runs dry.
The absence of liquid flowing through the turbine meter and the presence of only vapors commonly damages either the meter or the other system components whose operation is affected by the meter output. Problems have thus commonly arisen with respect to the use of the turbine meter in prior art anhydrous ammonia distribution systems. When the NH3 nurse tank runs empty, high velocity vapor passing through the turbine meter causes the impeller to spin at extremely high speeds. The meter bearings typically quickly fail or develop excessive wear, thereby causing flow reading errors. This cause for failure is present in any system with a turbine meter, even if the system is equipped with a heat exchanger to remove vapor. The heat exchanger requires liquid input to perform its intended operation, and when the nurse tank runs empty, only vapor flows through the heat exchanger and the turbine meter. Although turbine meters are thus widely used to measure the flow rate of anhydrous ammonia being applied by a farmer, these meters have high repair and maintenance costs.
The disadvantages of the prior art are overcome by the present invention, and an improved system for reliably measuring the flow rate of low temperature vaporization fluids, such as anhydrous ammonia, are provided so that flow may be reliably regulated in response thereto. The flow meter of the present invention is particularly well suited for use in measuring the flow rate of anhydrous ammonia which is applied to the field from a portable nurse tank.
The present invention may be used for controlling the flow of a low temperature vaporization liquid, such as NH3. The system is simple to install, easy to use and very accurate. The total gallons dispersed by the former agrees with the weigh scales within a very low percentage, typically about 1% or less. The desire to over applicate is eliminated with this system, thereby reducing the farmer""s fertilizer costs. Application rates of 220 pounds of NH3 per acre may be obtained using a 30 ft. tool bar at 5.5 miles/hr. with only 55 psi tank pressure. Since a heat exchanger and/or condenser is not required, the NH3 flow path has no added restrictions and more NH3 may be reliably applied in cold weather (early in the season, in the morning, etc.). The meter of the present invention preferably uses a variable area to determine flow rate and is immune to the high velocity vapor flow rates caused by a tank running empty. The variable area meter is simple, reliable, has excellent repeatability, and is rugged and trouble free. The system may include a vehicle having a control station with a tachometer, a throttle for adjusting the velocity of the vehicle, and a flow rate display. The control station may also include a toggle switch for adjusting the opening or closing of a valve in the flow line to regulate the flow through the system.
The present invention provides an apparatus and method for accurately controlling the amount of NH3 to be applied to a field. A vane is mounted in the flow stream and a spring holds the vane toward the fluid inlet. The measuring cavity in the meter is smaller on the inlet side and increases linearly toward the outlet side. As flow increases, the vane is pushed toward the outlet by the liquid flow until the force from the liquid is equal to the torque on the vane applied through the spring. The position of the vane is measured to determine the flow rate. A sensor in the liquid stream measures the liquid temperature to correct for density changes as the NH3 warms throughout the day. The flow rate may be measured when the NH3 is close to the nurse tank. Some vaporization typically has occurred prior to flow measurement, but the system provides a compensation to correct this error caused by vapor during flow measurement.
In a typical application, the vehicle may transport a tank containing NH3 and a tool bar for distributing the NH3 to the soil. A flow meter is mounted in the flow path downstream of the nurse tank withdrawal valve. After flowing through the flow meter, the NH3 is conducted to the soil through a series of hoses, fittings, a distributor, and tubing. The flow meter transmits a signal converted to a flow reading which is displayed on the control panel. The farmer is able to view and control the rate of application from the cab with high accuracy.
It is an object of the present invention to provide an improved system for measuring the flow rate of fluids and regulating flow rate in response thereto. The system of the present invention is particularly well suited for measuring the flow rate of low temperature vaporization liquids, such as anhydrous ammonia. The system of the present invention may thus be used in exemplary application by a farmer of fertilizing crops to accurately disperse liquid anhydrous ammonia from a nurse tank to the field.
It is another object of the present invention to provide an improved flow meter suitable for use in measuring the flow rate of a low temperature vaporization liquid. The flow meter is highly reliable and rugged, and may be manufactured and maintained at a relatively low cost.
The method of the invention may be used for controlling the amount of NH3 to be applied to a field from a nurse tank located on a vehicle. NH3 may be withdrawn through a valve located at an outlet of a tank, and the flow rate of the NH3 measured with a flow meter positioned downstream from the withdrawal valve. By comparing the measured NH3 total flow measured by the flow meter to the actual NH3 withdrawn from the nurse tank over a time interval, a correction factor may be devised for vapor in the measured NH3. In response to the measured flow rate and the derived correction factor, the actual NH3 flow rate may be determined at a control station located on the vehicle, and the amount of NH3 applied to the field adjusted in response to the determined flow rate.
It is an object of the invention to provide an improved system for controlling the amount of NH3 to be applied to a field from a nurse tank located on a vehicle. A control station on the vehicle is used for determining a ground speed of the vehicle. A flow meter downstream from the nurse tank output a flow rate signal to the control station, and a temperature sensor senses the NH3 temperature and outputs a temperature signal to the control station. Flow distribution lines downstream from the flow meter channel NH3 to a tool bar carried by the vehicle. The control station includes a constant valve correction device, such as a computer, for correcting the flow rate signal from the meter and the temperature signal from the temperature sensor. An output signal adjusts a valve located along the flow distribution line to regulate the flow of NH3.
It is another object of the invention to provide an improved variable area flow meter positionable along a flow line for sensing the flow rate of fluid through the flow line and outputting a signal in response thereto. The flow meter includes a housing having an internal cavity with a fluid inlet and a fluid outlet each for interconnection with the flow line, and a vane member rotatable about a shaft axis and moveable within the internal cavity in the housing to vary the flow area as a function of the flow rate through the meter. A magnet is fixedly positioned on the vane member within the cavity and rotatable with the vane member about the shaft axis. A spring or other biasing member biases the vane member in a preselected position. A sensor fixed to the housing exterior of the cavity outputs a signal in response to the position of the magnet with respect to the housing, with the signal being indicative of the rotational position of the vane member and thus the flow rate through the housing.
It is a feature of the present invention that the system may be used for measuring the flow of low temperature vaporization fluids without the use of a heat exchanger or other liquifier, thereby significantly reducing the system cost. Yet another feature of the invention is to provide a flow meter for use in a flow temperature vaporization system wherein the meter is not highly susceptible to damage when the source of liquid to the meter runs dry.
Yet another feature of the present invention is an improved system for regulating the flow rate of a low temperature vaporization fluid, wherein the compensation system allows the system to accurately measure the flow of liquid without the meter being positioned closely adjacent the liquid storage source. Yet another feature of the invention is a flow meter which need not be monitored at the particular position or have a particular orientation to provide a reliable output. The flow rate of the NH3 may be measured after the NH3 passes through a hose and a valve connected to the withdrawal valve of the nurse tanks. The amount of NH3 to be applied to the field may be controlled by adjusting either or both the flow rate of the NH3 or the ground speed of the vehicle.
Another feature of the invention is that the flow meter is not susceptible to damage due to high velocity vapor flowing through the meter. The flow meter of the present invention may provide a flow rate output which is compensated for temperature and thus varying density of the fluid, and also compensates for vapor in the fluid which is present when the flow measurement occurs.
It is an advantage of the present invention that the variable area meter is not adversely affected by hysterisis which is commonly associated with meters which utilize a magnetic coupling. The hall effect sensor is preferably used to output a signal indicative of the position of a vane shaft, thereby overcoming problems associated with sensors which use an LVDT or potentiometer. The output from the sensor varies in response to the position of a unitary magnet having a curvilinear configuration and fixedly positioned on the shaft of the vane member.
Yet another advantage of the system according to the present invention is that the meter is highly reliable and its operation is not adversely affected by debris in the flowing fluid.
Still another advantage of the present invention is that a GPS system may be used to monitor the speed of the vehicle and thereby accurately control the amount of the anhydrous ammonia applied to the given size of the field.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.